Recently, a battery formed using a nonaqueous solvent such as a lithium ion secondary battery has been developed as a battery having a high energy density. The lithium ion secondary battery is excellent in energy density and cycle characteristics compared to a lead storage battery and a nickel hydrogen secondary battery, and is expected to be used as a power source for large electricity storage, such as a power source for vehicles such as hybrid vehicles and electric vehicles. From the viewpoint of providing a large potential window, a nonaqueous solvent such as ethylene carbonate, diethyl carbonate or propylene carbonate is used as an electrolyte solution of the lithium ion secondary battery. Since these solvents are flammable, there are safety problems. If each of the solvents can be replaced with an aqueous electrolyte solution, it is possible to basically solve the problems. The aqueous electrolyte solution is inexpensive, compared to a nonaqueous electrolyte solution. Further, it is not necessary that a production process is performed in an inert atmosphere. Therefore, the nonaqueous electrolyte solution is replaced with the aqueous electrolyte solution, whereby a large cost reduction is expected.
However, there is a large problem in the use of the aqueous electrolyte solution for the lithium ion secondary battery. The problem is that the theoretical decomposition voltage calculated by the chemical equilibrium of water is 1.23 V, and thus a battery is configured to have a design voltage which is greater than the above voltage, whereby oxygen generates in a positive electrode and hydrogen generates in a negative electrode. In order to solve the problem, the oxygen overvoltage is increased at the oxidation side (positive electrode side) and the hydrogen overvoltage is increased at the reduction side (negative electrode side). From the viewpoint of kinetics, it is necessary to improve the design of the battery.
In the lithium ion secondary battery formed by using an aqueous solution, the charge/discharge process of the positive electrode is relatively easy. There are many reported examples of positive electrode active materials such as LiCoO2, LiMn2O4, and LiFePO4. Meanwhile, there are reported examples of negative electrode active materials having a lithium insertion/extraction potential of about −0.5 V (vs. SHE), such as LiTi2(PO4)3, TiP2O7, and VO2(B). However, there are few reported examples of active materials which operate at a potential lower than the above potential. This is due to the fact that it is difficult to charge/discharge because the generation of hydrogen proceeds in the negative electrode. Even when a battery is formed using each of the above-described active materials as the negative electrode active material, the average operating potential is less than 2 V. In this case, it is difficult to allow the battery to have a high energy density. Currently, there is no aqueous secondary battery which has an energy density greater than that of the lead storage battery or the nickel hydrogen secondary battery. For example, if a lithium ion secondary battery using an aqueous solution can be formed by using TiO2 or Li4Ti5O12 having a lower lithium insertion/extraction potential, it is possible to achieve a high energy density.